QTR-Quadtone RIP

I'm struggling with the math required to come up with the percentages
of black ink relative to light black.  On page 14 of the User Guide it
says to find the black patch that matches the 100% light black patch.
  Say it's 40%.  I then measure the 45% black patch and determine that
percentage needed to produce the same density of the 100% light black
and so on.  It says it's bit of simple math to figure the percentage
but I can't get my simple mind around it.  What is that formula?

Thanks,

Tom